Background
Climate alarmists claim that CO2-induced global warming will adversely impact the planet's freshwater resources by inducing large changes in global streamflow characteristics; and as a result, many scientists are examining long-term streamflow records in an attempt to see how temperature changes of the last several decades may -- or may not -- have impacted this aspect of the planet's hydrologic cycle, for in the words of Lins and Slack (1999), there is a widespread perception that "extreme hydrologic events are increasing in frequency and/or magnitude."

What was done
The authors studied discharge data they obtained from 34 stream gauges located in natural and anthropogenically-modified river basins of the Northern Rocky Mountains (USA) over the 59-year interval of 1950-2008, which period, in their words, "covers the majority of reported global climate change due to anthropogenic influences as defined by the Intergovernmental Panel on Climate Change's 2007 report, which concluded that climate change over the past half century has been greater than natural variability and is explained by anthropogenic forcing of the climate system."

What was learned
Arrigoni et al. determined that over the past 59 years "direct anthropogenic modifications of river basins by direct development (i.e., damming and irrigation) have substantially altered river flow regimes in the Northern Rocky Mountains," but they found that reported climate change in the western United States has not "generated statistically detectable changes in the flow regimes in the Northern Rocky Mountains over the period of record."

What it means
In the words of the three researchers, "changes in climate to date have not been great enough to significantly detect changes in the timing of flows in most natural sub-basins in the Northern Rocky Mountains beyond the natural variability," corroborating the findings of Moore et al. (2007). And as a result, they conclude that "direct anthropogenic modifications of river basins over the past 59 years have been more detrimental to overall river processes and ecosystem health than reported climate change effects in the Northern Rocky Mountains."

References
Lins, H.F. and Slack, J.R. 1999. Streamflow trends in the United States. Geophysical Research Letters 26: 227-230.

Moore, J.N., Harper, J.T. and Greenwood, M.C. 2007. Significance of trends towards earlier snowmelt runoff, Columbia and Missouri Basin headwaters, western United States. Geophysical Research Letters 34: 10.1029/2007GL031022.